The present invention relates to an improved apparatus, such as a nebulizer apparatus, for discharging fluids, and to a method of operating same. Nebulizers, or atomizers, are devices that generate a fine spray or aerosol, usually of a liquid. A particularly useful application for nebulizers is to provide a fine spray containing a dissolved or suspended particulate or colloidal pharmaceutical agent for administration to a subject by inhalation. Such inhalation treatment is highly effective for conditions affecting the subject's respiratory organs. Further, since the lungs are close to the heart and the circulatory system of the body, drug administration by inhalation provides an effective and rapid delivery system for a drug to all organs of the body. In other applications, nebulizers provide a fine spray of water for humidification.
When used to dispense a pharmaceutical agent to a subject, a nebulizer in the form of an inhaler may be placed directly in the mouth or nose of the subject so that the spray can be entrained in the respiratory gases which are inhaled during normal, spontaneous breathing of the subject.
In other applications, the subject breathes with the aid of a respiratory ventilator. A typical ventilator has a breathing circuit comprising an inhalation limb and an exhalation limb connected to two arms of a Y-connector. The third arm of the Y-connector is connected, via a patient limb, to a mouthpiece, mask, or endotracheal tube for the subject. The ventilator provides a complete or partial supply of breathing gas to the subject through the inhalation limb during inhalation. The contraction of the subject's lungs discharges gases through the exhalation limb during exhalation. When a nebulizer is employed in conjunction with a ventilator, it is typically placed in the patient limb to discharge into the breathing gases inhaled by the subject but it can also be placed in the inhalation limb of the breathing circuit.
Nebulizers are currently in use that generate the spray either pneumatically or by means of ultrasonic vibrations. Pneumatic nebulizers are typically used with a liquid, such as an aqueous drug solution. High pressure driving gas is conducted through a nozzle to draw the drug from a drug supply to the nebulizer. The drug is discharged against a baffle or other similar separating means in a gas space of the nebulizer, breaking the liquid into a fine spray. The gas space is in fluid communication with the inhaled gas pathway for the subject so that the gas flow expelled from the nozzle along with the nebulized drug is conducted from the gas space to the pathway and ultimately to the subject.
Disadvantages in the use of pneumatic nebulizers include the following. If the nebulizer adds a significant quantity of gas, for example, up to five (5) liters/minute, into the breathing gases, the overall breathing gas composition to the subject may be significantly altered. Further, due to passage of the driving gas through the nozzle, the impingement of the drug on the baffle, etc., pneumatic nebulizers tend to be noisy. And, controlling the commencing and stopping of a drug spray is difficult and is not very accurate. This may result in wastage of the drug.
The foregoing shortcomings of pneumatic nebulizers have led to the use of ultrasonic nebulizers in which a fine spray is produced by ultrasonic vibration of the liquid containing the drug, as through the use of a piezoelectric crystal. The breathing gas composition and the on-off operation are easier to control with ultrasonic nebulizers than with a pneumatic nebulizer. However, ultrasonic devices require a large, bulky electrical power supply to power the crystal and may not be able to nebulizer colloidal or particulate suspensions.
In one type of ultrasonic nebulizer, the fine spray is produced by dropping the liquid on, or otherwise applying it to, the vibrating element. See Koeh et al. U.S. Pat. No. 5,443,059. Michaels et al., U.S. Pat. No. 3,812,854, describe another type of nebulizer for inhalation therapy in which the spray is generated on the front surface of a vibrating, porous body. The pores in the body form a network of passages that enable the liquid to flow through the body. The liquid to be nebulized is supplied under pressure from a liquid supply through a liquid conduit to the pores, and forced through the pores to the front surface of the porous body where it is discharged as a spray. Robertson et al., U.S. Pat. No. 5,487,378, describe a nebulizer in which the aerosol is formed using a mesh plate instead of a porous solid body. The mesh plate has a plurality of orifices through which the liquid can pass. Either the liquid or the mesh plate is vibrated ultrasonically by a piezoelectric element to nebulizer the liquid as it passes through the mesh plate.
A general shortcoming of current nebulizers is the efficiency with which the aerosol is transported into the subject's lungs. To increase the efficiency, the nebulizer may be operated so as to function in phase with the ventilator so that the aerosol is produced by the nebulizer either during, or partly during, inspiration by the subject. The proper timing can be achieved by switching the nebulizer on at the beginning of inspiration in response to a signal coming from the ventilator, or in response to information coming from a flow sensor as described in U.S. Pat. No. 5,964,219 to Pekka Merilainen.
However, a disadvantage currently exists in that to ensure that the nebulizer functions properly to achieve the best efficiency, a separate device is required to generate the necessary signal or information for optimal timing of the nebulization. This adds to the cost and complexity of the nebulizer and/or ventilator. For example, Ivri et al., U.S. Pat. No. 6,085,740, describe a nebulizer of the inhaler type in which the inhalation flow is detected from an audible signal produced during inhalation, which signal is then used to control the nebulization. In addition to cost and complexity, this approach may exhibit a sensitivity to external noise.